The effects of lidocaine, propranolol and quinidine on the characteristics of a type of action potential called a slow response will be determined. Slow responses are abnormal action potentials which can be induced in cardiac cells by various conditions which lead to cardiac arrhythmias. Thus, it is believed that development of slow responses may be involved in production of arrhythmias in the intact heart. Determination of the effects of proven antiarrhythmic drugs on slow responses induced by low extracellular pH (6.0) in isolated canine cardiac Purkinje fibers will be done. In a related study the influence of alterations in extracellular potassium concentration combined with low extracellular pH will be determined on the transmembrane potential of isolated canine cardiac fibers.